There have been many proposals in the past of methods and apparatus for oxygen bleaching of pulp at various pulp consistencies. For instance, U.S. Pat. No. 3,660,225 increases the consistency of the digested pulp to about 20 to 30 percent, fluffs it, and moves it through successive trays in a reaction vessel establishing the necessary retension time and maintaining small bed heights to prevent pulp compression so that the oxygen may be defused to contact all of the fibers. While such a structure is successful, it is relatively expensive since concentrating apparatus must be employed, expensive rotating structures are utilized, and bed compression can result as the pulp drops from an upper bed to a lower bed. Another type of prior art oxygen bleaching equipment, such as shown in U.S. Pat. Nos. 3,138,947 and 3,963,561, provides for a passage of the pulp and oxygen mixture upwardly in a central tube of the reaction vessel, and then downwardly over the top of the central tube into the annulus between the central tube and the pressure vessel. Severe channelling can take place in such structures, some of the pulp moving through the entire vessel much too rapidly so that there is only partial bleaching thereof, while other portions moving through the vessel much too slowly with resultant degradation of the cellulose fibers.
According to the present invention, the retention time for each individual portion of the pulp within the reacting vessel is more accurately controlled, channelling being substantially reduced, while a continuous feed and discharge are maintained. According to the method of the present invention, oxygen bleaching of cellulosic pulp at super atmospheric pressure and temperature is effected utilizing a first pressurized chamber, a plurality of generally prismatic hollow members (i.e. tubes) in fluid communication with the first chamber at one end thereof, and a second pressurized chamber, at least some of the hollow members in fluid communication with the second chamber at the other ends thereof. The method comprises the steps of mixing oxygen bearing fluid with digested cellulosic pulp and continuously feeding the pulp under pressure to the first chamber. Then, selective communication is provided between the first and second chambers through at least one of the members to allow pulp passage from the member to the second chamber, while blocking passage between others of the members and the second chamber. The method comprises further steps of progressively changing at least one member through which pulp passes to the second chamber and the members through which passage is blocked, so pulp remains within each member sufficient time to allow oxygen-bleaching thereof without degradation, and passes through the members without channeling, and continuously discharging the oxygen bleached pulp from the second chamber. The pulp normally has a consistency of about 6 to 15 percent, and passage between the first and second chambers may be either vertically upwardly or vertically downwardly.
Oxygen-bleaching apparatus according to the present invention comprises a pressurized first chamber adapted to receive digested cellulosic pulp and including an inlet, and means for adding an oxygen bearing fluid to digested cellulosic pulp and mixing the oxygen with the pulp before passage into the inlet. A plurality of generally prismatic hollow members each having first and second ends are also provided, each in fluid communication at the first end thereof with the first chamber and extending therefrom. The apparatus further includes a pressurized second chamber adapted to receive pulp therein, the second chamber being in fluid communication with at least some of the said members at the second end thereof, the second chamber including an outlet for oxygen bleached pulp; means for selectively providing passage of digested cellulosic pulp mixed with oxygen through at least one of the members from at least one member to the second chamber while preventing passage of the pulp between others of the plurality of members and the second chamber; and means for progressively moving the passage providing and blocking means so that it provides communication between each member and the second chamber in turn, so that pulp remains within each member sufficient time to allow oxygen bleaching thereof without degradation. The members are preferably a plurality of tubes extending vertically between the first and second members and disposed in one or more circles, with the passage providing and blocking means including a rotatable plate having an aperture therein for in-turn communication with each of the tubes during rotation of the plate. The rotation of the plate may be continuous or it may be indexed from tube to tube, with rapid movement between the tubes. A scraper is provided in the second chamber to facilitate discharge of the oxygen bleached pulp through the outlet.
It is the primary object of the present invention to provide a relatively inexpensive method and apparatus for continuous oxygen bleaching of pulp, without channelling. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.